1. Field of the Invention
The present invention relates to a band switch control apparatus for an intermediate frequency filter.
2. Description of the Related Art
With regard to an intermediate frequency filter (hereinafter referred to as IF filter) extracting an intermediate frequency component of a reception signal, there has been proposed a receiving apparatus employing a band switching system of switching the pass bandwidth of the IF filter based on an adjacent interference signal and a modulation degree (see Japanese Patent Application Laid-Open Publication No. 2003-143025).
FIG. 17 is a flowchart of an operation flow of the receiving apparatus.
Based on a reception signal (frequency modulated signal) received from an antenna, an adjacent interference signal A superimposed on the reception signal and a modulation degree M of the reception signal are detected (S1701), and amplitude (level) of the adjacent interference signal A is first compared with a predetermined threshold value Ath1 (S1702). If the adjacent interference signal A is greater than the threshold value Ath1 (S1702: YES), the amplitude of the adjacent interference signal A is further compared with a predetermined threshold value Ath2 greater than the threshold value Ath1 (S1703). If the amplitude of the adjacent interference signal A is smaller than the threshold value Ath1, the band broadening control is performed to broaden the pass bandwidth Bw of the IF filter step-by-step within a range from the minimum value Bw1 to the maximum value Bw3 (Bw1→Bw2→Bw3) (S1704). A reason why the pass bandwidth Bw of the IF filter is narrowed when the adjacent interference signal A is increased is that if the pass bandwidth Bw of the IF filter is broadened when the adjacent interference signal A is increased, interference tends to occur.
If the amplitude of the adjacent interference signal A is greater than the threshold value Ath2 (S1703: YES), a first bandwidth Bw1 is selected as the pass bandwidth Bw of the IF filter (S1705). If the amplitude of the adjacent interference signal A is smaller than the threshold value Ath2 (S1703: NO), the modulation degree M is compared with a predetermined threshold value Mth (S1706). If the modulation degree M is greater than the threshold value Mth (S1706: YES), a second bandwidth Bw2 greater than the first bandwidth Bw1 is selected as the pass bandwidth Bw of the IF filter (S1707). If the modulation degree M is smaller than the threshold value Mth (S1706: NO), the first bandwidth Bw1 is selected as the pass bandwidth Bw of the IF filter (S1705). A reason why the pass bandwidth Bw of the IF filter is broadened when the modulation degree M is increased is that if the pass bandwidth Bw is narrowed when the modulation degree M is increased, great amount of the FM detected audio signal component is lost and thus the audio signal waveform tends to be distorted.
In a table of FIG. 18, there is listed the pass bandwidth Bw of the IF filter to be selected according to the amplitude of the adjacent interference signal A and the modulation degree M in accordance with the flowchart shown in FIG. 17. If the adjacent interference signal A is equal to or greater than the threshold value Ath1 and smaller than the threshold value Ath2 and the modulation degree M is greater than the threshold value Mth, for example, the bandwidth switching according to the modulation degree M is prioritized to select the second bandwidth Bw2 greater than the first bandwidth Bw1, and thus generation of the waveform distortion in the audio signal is suppressed. On the other hand, if the modulation degree M is smaller than the threshold value Mth, the bandwidth switching according to the adjacent interference signal A is prioritized to select the first bandwidth Bw1 narrower than the second bandwidth Bw2, and thus generation of interference is suppressed. In this way, by changing the pass bandwidth Bw based on the amplitude of the adjacent interference signal A and the modulation degree M, there can be achieved a receiving condition with less waveform distortion in the audio signal or less interference.
As described above, if the adjacent interference signal A is equal to or greater than the threshold value Ath1 and smaller than the threshold value Ath2 and the modulation degree M is less than the threshold value Mth, for example, there is selected the first bandwidth Bw1 that is the smallest as the pass bandwidth Bw. However, in this case, since the amplitude of the adjacent interference signal A is less than the threshold value Ath2, the first bandwidth Bw1, which is the narrowest as the pass bandwidth Bw, is not required to be selected for preventing the interference due to the adjacent interference signal A, so that the second bandwidth Bw2 broader than the first bandwidth Bw1 is practically adequate to be select. In the conventional band switching, if the modulation degree M is small, the bandwidth of the IF filter is always reduced even when the effect of the adjacent interference is small. Therefore, the audio quality may be deteriorated under the condition that the effect of the adjacent interference is small.
Since a time constant practically exists in the detection of the modulation degree M, for example, even if the modulation degree M changes from a state of a value less than the threshold value Mth to a momentary state of a value exceeding the threshold value Mth, the momentary state may not be detected. In this case, when the modulation degree M is momentarily increased, the pass bandwidth Bw is normally required to be increased to restrain the waveform distortion in the audio signal. However, the momentary state in the modulation degree M may not be detected due to the time constant, to delay the band broadening control, so that the bandwidth change is not made from the first bandwidth Bw1, which is the narrowest, resulting in the waveform distortion in the audio signal.